This invention relates to a Purex wet process, or a method of reprocessing spent nuclear fuel, and more particularly to an improved method of recovering neptunium which is one of the transuranium elements contained in spent nuclear fuel.
The primary object of the Purex wet process is to separate uranium and plutonium contained in spent nuclear fuel from the fission products. Heretofore, neptunium, which is one of the transuranium elements contained in spent nuclear fuel, has not been included among the main object matters to be recovered in the Purex wet process, and it has been either separated together with uranium and plutonium from the fission products or together with the fission products from uranium and plutonium. Neptunium per se has a long half-value period of decay and hence can not be utilized immediately as a radiation source. Also, as it is not a fissile material by nature, it can not be reused as nuclear fuel. However, neptunium can become a parent element of plutonium-238 which is produced by neutron irradiation in for instance a nuclear reactor. This plutonium-238 is very useful as radioisotope which has the highest utility value among the various radiation sources. Therefore, it is lately attempted to efficiently separate and recover neptunium in the Purex wet process of spent nuclear fuel. For efficient separation and recovery of neptunium, there is usually employed a method in which neptunium is separated from the flow of uranium and plutonium while preventing, as much as possible, neptunium from flowing out into the highly radioactive waste liquid of high beta-ray and gamma-ray concentration produced by fission products. It is necessary to minimize complication of the equipments and treating method under thick gamma-ray protective shield. Also, recovery of neptunium should not impair separation of uranium and plutonium from the fission products, which is the primary object of the Purex wet recovery process.
The Purex wet process is a method in which the spent nuclear fuel is dissolved in nitric acid and uranium and plutonium are selectively extracted in tributyl phosphate (hereinafter referred to as TBP) which is an excellent solvent for the extraction of uranium and plutonium, and then these elements are separated from the fission products.
A typical example of the conventional Purex wet recovery process is systematically illustrated in FIG. 1. Spent nuclear fuel undergoes a pre-treatment and then is dissolved in nitric acid, and then the mixed solution, after being adjusted in its composition, is subjected to a solvent extraction treatment. The solvent extraction treatment is roughly divided into four steps: decontaminating step, distributing step, uranium refining step, and plutonium refining step. In the treatment, uranium and plutonium are adjusted to an atomic valency suited for processing in each step. In the decontamination step where first and second extractors are used, uranium and plutonium are extracted with a (VI) valence and a (IV) valence, respectively, by TBP in the first extractor and separated from the fission products, and then in the second extractor, both uranium and plutonium which have thus been extracted in the TBP phase, are back-extracted in an aqueous liquid phase with the same valency. In the drawings, the fission products are denoted by F.P. and double parentheses (( )) indicate that the element in such double parentheses exists only in a minute amount. In the distribution step which includes third, fourth and fifth extractors, the aqueous solution of uranium and plutonium which have been back-extracted in the second extractor is supplied into the third extractor where both uranium and plutonium are again extracted with TBP under the same valency to further expedite separation from the fission products. In the fourth extractor, although uranium is maintained at a (VI) valence, plutonium is adjusted to a (III) valence, and the former is distributed in the TBP phase while the latter in the aqueous liquid phase. The TBP-phase uranium, while kept at a (VI) valence, is back-extracted into the aqueous liquid phase in the fifth extractor. The uranium refining step comprises the sixth and seventh extractors in both of which uranium is extracted and back-extracted while maintained at a (VI) valence and separated from plutonium (which exists mixed at a small concentration) and the fission products. In the plutonium refining step which includes eighth and ninth extractors, plutonium which has previously been distributed in the aqueous liquid phase in the fourth extractor is adjusted to a (IV) valence, then extracted with such valence in the eighth extractor and then recovered from the nineth extractor in the form of a highly refined aqueous solution adjusted to a (III) valence.
Thus, in the conventional Purex wet process, neptunium which exists mixed with uranium and plutonium has not been involved among the main object substances to be recovered, and it has been either separated into the waste liquid together with the fission products or separated from the fission products together with uranium and plutonium, and in the latter case, neptunium has not been ultimately recovered but separated into the waste liquid.